Silicon-based (SOI) material is a new type of material for silicon-based integrated circuits and optoelectronic integrated circuits. This material is highly valued by major integrated circuit manufacturers in the world for its high integration, excellent high-frequency characteristic, low power consumption, good compatibility with existing large scale integrated circuit technologies, and it is also the most competitive integrated material in high-speed optical communication system at present. A variety of silicon-based photonic devices such as high-speed electro-optical modulators, photodetectors, and integrated chips have emerged and matured gradually.
The difference between the refractive index of silicon dioxide and that of a silicon waveguide made of the SOI material is large, which greatly enhances the limiting effect of a waveguide layer on light and may reduce the size of the waveguide to the micron order or less, enabling an unparalleled advantage for the SOI material in integration. However, the fiber core of an optical fiber is mainly made of silicon dioxide, and such mismatch between the refractive index of silicon dioxide and that of the SOI material waveguide will directly lead to a relatively great optical loss in coupling alignment between the optical fiber and the silicon waveguide.
In order to solve the problem of great coupling loss, a variety of mode field conversion structures for increasing the mode field diameter of the silicon waveguide have emerged, mainly including a forward wedge mode field conversion structure, a reverse wedge mode field conversion structure, a prism coupler, a grating coupler, and so on. The reverse wedge mode field conversion structure is applied in the field of integrated optical devices due to its advantages of high coupling efficiency, large alignment tolerance, easy integration, and so on. Using this structure, the size of the silicon waveguide is reduced, so that the limiting effect on the mode field is decreased in center of the waveguide and a large amount of mode field leaks into a chip cladding which has a refractive index similar to that of the fiber core, thereby the matching with the refractive index of the fiber core is obtained. In order to prevent great loss of the optical power caused by the mold field re-entering a silicon substrate layer through the under-cladding of the chip, the silicon around the cladding used as the mode field conversion structure is usually etched away. FIG. 1 and FIG. 2 are structural schematic diagrams of two suspended waveguides 1. Such suspended waveguide has a cross-section diameter of only several microns, thus it is very fragile and cannot withstand a relatively high stress. During the coupling alignment with an optical fiber, as shown in FIG. 3, the spacing between the optical fiber and the waveguide is set to be very small in order to obtain a relatively small coupling loss. A high impacting contact force will be easily produced between the fiber and the waveguide due to a slight carelessness, which will cause the damage of the suspended waveguide.